Imaging optical system

ABSTRACT

An imaging optical system includes sequentially from an object side a front group configured to include a positive lens disposed at a position nearest a diaphragm; the diaphragm; and a rear group configured to include a negative lens disposed at a position nearest the diaphragm. The imaging optical system satisfies a conditional expression (1) 0.27≦|θ3/θ2|≦1.80, where θ3 represents temperature-dependent variation of relative refractive index for d-line of the negative lens and θ2 represents temperature-dependent variation of relative refractive index for d-line of the positive lens.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an imaging optical system.

2. Description of the Related Art

There is a need for surveillance cameras and vehicle cameras to be compact and for imaging optical systems for such cameras to be smaller since space for mounting is often limited. Furthermore, surveillance cameras and vehicle cameras are often used at night, requiring a bright imaging optical system. In response to such demands, numerous imaging optical systems have been proposed that can be mounted on surveillance cameras, vehicle cameras, etc. (for example, refer to Japanese Patent Application Laid-Open Publication Nos. 2013-92774, 2013-47753, and 2008-8960).

The optical system disclosed in Japanese Patent Application Laid-Open Publication No. 2013-92774 is configured by five lenses and has an F number on the order of 2.4. The optical systems disclosed in Japanese Patent Application Laid-Open Publication Nos. 2013-47753 and 2008-8960 are configured by five lenses and have an F number of 2.0.

In recent years, surveillance cameras and vehicle cameras have come down in price and lower-cost imaging optical systems for such cameras are also demanded. Further, with the rapid increases in the pixel density of solid state image sensors (CCD, CMOS, etc.), bright, high-resolution imaging optical systems capable of supporting solid state image sensors with high pixel densities have come to be demanded.

Surveillance cameras are often installed outdoors, where the temperature varies greatly. Further, vehicle cameras are installed inside vehicles, where especially during the summer, the temperature becomes very high. Thus, among imaging optical systems for surveillance cameras and vehicle cameras, an imaging optical system that can maintain high resolution over a wide temperature range, from low temperatures to high temperatures, is demanded. In particular, an optical system having a deep depth of focus is more advantageous in preventing focus errors during extreme temperatures. Further, if the depth of focus is deep, drops in image quality consequent to shifts in lens centers at the time of product assembly can also be prevented.

The optical system described in Japanese Patent Application Laid-Open Publication No. 2013-92774 is configured by five lenses and therefore, has a low manufacturing cost and is also a bright lens having an F number of 2.4. Nonetheless, resolution is low because various types of aberration are not sufficiently corrected, making high quality images difficult to obtain. In particular, since the depth of focus is shallow, focus errors occur easily with extreme temperatures, making high resolution difficult to maintain over a wide temperature range, from low temperatures to high temperatures. If the depth of focus is shallow, a further problem arises in that image quality is easily affected by shifts in lens centers occurring at the time of product manufacturing.

The optical system described in Japanese Patent Application Laid-Open Publication No. 2013-47753 is also configured by five lenses and therefore, has a low manufacturing cost and is a bright lens having an F number of 2.0. Nonetheless, resolution is low because various types of aberration are not sufficiently corrected, making high quality images difficult to obtain. Further, since the temperature coefficient of refractive index of the glass lens material is large, focus errors during extreme temperatures are large, making high resolution difficult to maintain over a wide temperature range, from low temperatures to high temperatures.

The optical system described in Japanese Patent Application Laid-Open Publication No. 2008-8960 is also configured by five lenses and therefore, has a low manufacturing cost and is a bright lens having an F number of 2.0. Nonetheless, resolution is low because various types of aberration are not sufficiently corrected, making high quality images difficult to obtain. Further, similar to the optical system described in Japanese Patent Application Laid-Open Publication No. 2013-92774, since the depth of focus is shallow, focus error occurs easily with extreme temperatures, making high resolution difficult to maintain over a wide temperature range, from low temperatures to high temperatures. If the depth of focus is shallow, a further problem arises in that image quality is easily affected by shifts in lens centers occurring at the time of product manufacturing.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the above problems in the conventional technologies.

An imaging optical system includes sequentially from an object side a front group configured to include a positive lens disposed at a position nearest a diaphragm; the diaphragm; and a rear group configured to include a negative lens disposed at a position nearest the diaphragm.

The imaging optical system satisfies a conditional expression (1) 0.27≦|θ3/θ2|≦1.80, where θ3 represents temperature-dependent variation of relative refractive index for d-line of the negative lens and θ2 represents temperature-dependent variation of relative refractive index for d-line of the positive lens.

The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting, along an optical axis, a configuration of the imaging optical system according to a first embodiment;

FIG. 2 is a diagram of various types of aberration occurring in the imaging optical system according to the first embodiment;

FIG. 3 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a second embodiment;

FIG. 4 is a diagram of various types of aberration occurring in the imaging optical system according to the second embodiment;

FIG. 5 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a third embodiment;

FIG. 6 is a diagram of various types of aberration occurring in the imaging optical system according to the third embodiment;

FIG. 7 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a fourth embodiment;

FIG. 8 is a diagram of various types of aberration occurring in the imaging optical system according to the fourth embodiment;

FIG. 9 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a fifth embodiment;

FIG. 10 is a diagram of various types of aberration occurring in the imaging optical system according to the fifth embodiment;

FIG. 11 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a sixth embodiment; and

FIG. 12 is a diagram of various types of aberration occurring in the imaging optical system according to the sixth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of an imaging optical system according to the present invention will be described in detail with reference to the accompanying drawings.

During high or low temperatures, the refractive index of a lens may vary consequent to expansion or contraction of the lens. Variation of the refractive index leads to focus errors in the optical system and drops in resolution. Therefore, to maintain high resolution over a wide temperature range from low temperatures to high temperatures, it is particularly important to properly control refractive index variation of the lens during extreme temperatures and to suppress focus errors of the optical system.

Thus, the imaging optical system according to the present invention is configured to include sequentially from the object side, a front group, a diaphragm, and a rear group. Further, a positive lens is disposed in the front group, at a position nearest the diaphragm, a negative lens is disposed in the rear group, at a position nearest the diaphragm, and the following condition expression is preferably satisfied, where θ3 represents temperature-dependent variation of the relative refractive index for d-line of the negative lens and θ2 represents temperature-dependent variation of the relative refractive index for d-line of the positive lens. The temperature-dependent variation of the relative refractive index is defined by the temperature-dependent variation of the refractive index of air of the same temperature as the lens material.

0.27≦|θ3/θ2|≦1.80  (1)

Conditional expression (1) represents a condition for suppressing focus error during extreme temperatures. Satisfying conditional expression (1) enables focus error to be suppressed during extreme temperatures and high resolution to be maintained over a wide temperature range, from low temperatures to high temperatures. In the present invention, temperature-dependent variation of relative refractive index for d-line of the lens disposed nearest the diaphragm and most easily influencing drops in resolution consequent to temperature variation is prescribed.

Below the lower limit of conditional expression (1), temperature-dependent variation of the relative refractive index for d-line of the material that can be used for the positive lens disposed in the front group, at a position nearest the diaphragm, becomes too great, whereby focus error during extreme temperatures increases and resolution drops. Meanwhile, above the upper limit of conditional expression (1), temperature-dependent variation of the relative refractive index for d-line of the material that can be used for the positive lens disposed in the front group, at a position nearest the diaphragm becomes too small, whereby focus error during extreme temperatures increases and resolution drops.

An even more desirable effect can be expected by satisfying conditional expression (1) within the following range.

0.49≦|θ3/θ2|≦1.45  (1a)

Satisfying the range prescribed by conditional expression (1a) enables higher resolution to be maintained even with extreme temperatures.

In the imaging optical system according to the present invention, to favorably correct spherical aberration, the negative lens disposed in the rear group at a position nearest the diaphragm preferably has a concave surface on the object side. If the surface on the object side of the negative lens is convex, the occurrence of spherical aberration becomes conspicuous, leading to drops in resolution.

Further, to prevent focus error from occurring with extreme temperatures, the range of the depth of focus is increased, i.e., the depth of focus of the optical system is made deep.

Thus, the imaging optical system according to the present invention preferably satisfies the following conditional expressions, where F2 represents the focal length of the rear group, f21 represents the focal length of the negative lens disposed in the rear group, at a position nearest the diaphragm, F1 represents the focal length of the front group, and f12 represents the focal length of the positive lens disposed in the front group, at a position nearest the diaphragm.

−2.4≦F2/f21≦−1.3  (2)

1.00≦F1/f12≦1.65  (3)

Conditional expressions (2) and (3) represent conditions to make the depth of focus of the optical system deep. Satisfying conditional expressions (2) and (3) enables an imaging optical system having a deep depth of focus to be realized. If the depth of focus is deep, the occurrence of focus error is suppressed even during extreme temperatures and high resolution can be maintained over a wide temperature range, from low temperatures to high temperatures. Further, if the depth of focus is deep, image quality is not easily affected by shifts in lens centers occurring at the time of product manufacturing, enabling favorable image quality to be maintained.

Below the lower limit of conditional expression (2), the correction of spherical aberration is insufficient and the depth of focus becomes shallow. Meanwhile, above the upper limit of conditional expression (2), spherical aberration is over corrected and resolution drops.

An even more desirable effect can be expected by satisfying conditional expression (2) within the following range.

−2.04≦F2/f21≦−1.51  (2a)

Satisfying the range prescribed by conditional expression (2a) enables spherical aberration to be corrected more favorably and the depth of focus of the optical system to be deep.

Below the lower limit of conditional expression (3), spherical aberration becomes over corrected and resolution drops. Meanwhile, above the upper limit of conditional expression (3), the correction of spherical aberration is insufficient and the depth of focus becomes shallow.

An even more desirable effect can be expected by satisfying conditional expression (3) within the following range.

1.25≦F1/f12≦1.54  (3a)

Satisfying the range prescribed by conditional expression (3a) enables spherical aberration to be corrected more favorably and the depth of focus of the optical system to be deep.

In the imaging optical system according to the present invention, the rear group preferably includes sequentially from the object side, a first lens, a second lens, and a third lens, and the following conditional expression is preferably satisfied where, υ22 represents the Abbe number for d-line of the second lens and υ21 represents the Abbe number for d-line of the first lens.

3.6≦υ22/υ21≦5.6  (4)

Conditional expression (4) represents a condition for suppressing chromatic aberration of magnification and astigmatism. Satisfying conditional expression (4) enables chromatic aberration of magnification and astigmatism to be favorably corrected and high resolution to be maintained.

Below the lower limit of conditional expression (4), dispersion of the second lens of the rear group becomes too small, chromatic aberration of magnification becomes difficult to correct, and resolution drops. Meanwhile, above the upper limit of conditional expression (4), astigmatism becomes difficult to correct and resolution drops.

An even more desirable effect can be expected by satisfying conditional expression (4) within the following range.

3.9≦υ22/υ21≦5.2  (4a)

Satisfying the range prescribed by conditional expression (4a) enables chromatic aberration of magnification and astigmatism to be more favorably corrected.

In the imaging optical system according to the present invention, the rear group preferably includes sequentially from the object side, a first lens, a second lens, and a third lens, and the following conditional expression is preferably satisfied, where f23 represents the focal length of the third lens and F1 represents the focal length of the front group.

1.15≦|f23/F1|≦3.0  (5)

Conditional expression (5) represents a condition for favorably correcting distortion and preventing drops in peripheral illumination. Satisfying conditional expression (5) enables distortion to be favorably corrected, drops in peripheral illumination to be prevented, and high resolution to be maintained.

Below the lower limit of conditional expression (5), the power of the front group becomes too weak with respect to the rear group, distortion becomes difficult to correct, and resolution drops. Meanwhile, above the upper limit of conditional expression (5), the refractive power of the third lens of the rear group becomes too weak, the incident angle of the principal ray to the image plane becomes too large, and peripheral illumination and resolution drop.

An even more desirable effect can be expected by satisfying conditional expression (5) within the following range.

1.56≦|f23/F1|≦2.64  (5a)

Satisfying the range prescribed by conditional expression (5a) enables improved resolution.

In the imaging optical system according to the present invention, as described above, the rear group is configured by three lenses, however, the number of lenses configuring the front group is not specified. In other words, although the front group has to have at least one lens, the front group may be configured by two lenses, or by three lenses. However, if the number of lenses is reduced, various types of aberration become difficult to correct and focus error during extreme temperatures increases, leading to drops in resolution. Therefore, to maintain high resolution while suppressing manufacturing cost, the front group is preferably configured by two lenses.

As described, the imaging optical system according to the present invention has the configuration above whereby, properly prescribing temperature-dependent variation of the relative refractive index of the lenses enables an imaging optical system that can maintain high resolution over a wide temperature range, from low temperatures to high temperatures to be provided at a low cost.

In addition, an imaging optical system having a deep depth of focus can be provided. If the depth of focus is deep, the occurrence of focus error is suppressed during extreme temperatures and high resolution can be maintained over a wide temperature range, from low temperatures to high temperatures. Further, if the depth of focus is deep, image quality is not easily affected by shifts in lens centers occurring at the time of product manufacturing.

The imaging optical system of the present invention having the characteristics described is particularly suitable for surveillance cameras, vehicle cameras, etc. used under conditions of extreme environmental temperature changes.

Embodiments of the imaging optical system according to the present invention will be described in detail with reference to the accompanying drawings. The present invention is not limited by the embodiments.

FIG. 1 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a first embodiment. The imaging optical system is configured to include sequentially from the object side nearest a non-depicted object, a front group G₁₁ having a positive refractive power, a diaphragm STP prescribing a given aperture, and a rear group G₁₂ having a positive refractive power. Between the rear group G₁₂ and an image plane IMG, an optical filter F and a cover glass CG are disposed sequentially from the object side. At the image plane IMG, the light receiving surface of a solid state image sensor is disposed.

The front group G₁₁ is configured to include sequentially from the object side, a negative lens L₁₁₁ and a positive lens L₁₁₂. Both surfaces of the positive lens L₁₁₂ are aspheric.

The rear group G₁₂ is configured to include sequentially from the object side, a negative lens L₁₂₁ (first lens), a positive lens L₁₂₂ (second lens), and a positive lens L₁₂₃ (third lens). The surface on the object side of the negative lens L₁₂₁ is concave. Both surfaces of the positive lens L₁₂₃ are aspheric.

Here, various types of data related to the imaging optical system according to the first embodiment are given.

f (focal length of entire imaging optical system) = 6.0 Fno. (F number) = 2.0 2ω (angle of view) = 60.0 (Lens data) r₁ = 14.6435 d₁ = 1.3224 nd₁ = 1.658 υd₁ = 50.85 r₂ = 2.6278 d₂ = 2.2613 r₃ = 5.0017 (aspheric) d₃ = 2.0875 nd₂ = 1.851 υd₂ = 40.10 r₄ = −11.3152 (aspheric) d₄ = 0.0759 r₅ = ∞ (diaphragm) d₅ = 1.9704 r₆ = −2.4167 d₆ = 0.6151 nd₃ = 1.946 υd₃ = 17.98 r₇ = −5.3862 d₇ = 0.7218 r₈ = −4676.3070 d₈ = 1.2966 nd₄ = 1.593 υd₄ = 68.62 r₉ = −9.0585 d₉ = 0.0772 r₁₀ = 9.3738 (aspheric) d₁₀ = 1.8418 nd₅ = 1.592 υd₅ = 67.02 r₁₁ = −9.1137 (aspheric) d₁₁ = 0.1000 r₁₂ = ∞ d₁₂ = 1.0000 nd₆ = 1.516 υd₆ = 64.14 r₁₃ = ∞ d₁₃ = 5.7000 r₁₄ = ∞ d₁₄ = 0.4000 nd₇ = 1.516 υd₇ = 64.14 r₁₅ = ∞ d₁₅ = 0.1512 r₁₆ = ∞ (image plane) Constant of the cone (ε) and aspheric coefficients (A, B, C, D, E) (Third order) ε = 1, A = 5.45426 × 10⁻⁴, B = −5.10209 × 10⁻⁶, C = −1.92183 × 10⁻⁵, D = 1.33392 × 10⁻⁷, E = −8.97359 × 10⁻⁸ (Fourth order) ε = 1, A = −2.49645 × 10⁻³, B = −5.92804 × 10⁻⁶, C = −9.19254 × 10⁻⁶, D = 1.34117 × 10⁻⁶, E = −1.08535 × 10⁻⁷ (Tenth order) ε = 1, A = 1.82213 × 10⁻⁴, B = 6.38939 × 10⁻⁵, C = 2.67778 × 10⁻⁶, D = −5.92153 × 10⁻⁷, E = 6.62019 × 10⁻⁹ (Eleventh order) ε = 1, A = 2.62777 × 10⁻³, B = 5.31356 × 10⁻⁵, C = 9.09030 × 10⁻⁶, D = −1.10441 × 10⁻⁶, E = 1.74049 × 10⁻⁸ (Values related to conditional expression (1)) θ3 (temperature-dependent variation of relative refractive index for d-line of negative lens L₁₂₁) = 3.7 θ2 (temperature-dependent variation of relative refractive index for d-line of positive lens L₁₁₂) = 7.5 |θ3/θ2| = 0.49 (Values related to conditional expression (2)) F2 (focal length of rear group G₁₂) = 12.08 f21 (focal length of negative lens L₁₂₁) = −5.15 F2/f21 = −2.34 (Values related to conditional expression (3)) F1 (focal length of front group G₁₁) = 6.87 f12 (focal length of positive lens L₁₁₂) = 4.33 F1/f12 = 1.59 (Values related to conditional expression (4)) υ22 (Abbe number for d-line of positive lens L₁₂₂ (second lens)) = 68.62 υ21 (Abbe number for d-line of negative lens L₁₂₁ (first lens)) = 17.98 υ22/υ21 = 3.81 (Values related to conditional expression (5)) f23 (focal length of positive lens L₁₂₃ (third lens)) = 8.11 F1 (focal length of front group G₁₁) = 6.87 |f23/F1| = 1.18

FIG. 2 is a diagram of various types of aberration occurring in the imaging optical system according to the first embodiment. In the diagram, for the curve depicting spherical aberration, the vertical axis represents the F number (Fno.) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the solid line depicts characteristics of the sagittal plane (S) and the broken line depicts characteristics on the meridonal plane (M). For the curve depicting distortion, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (κ=587.56 nm) are depicted. For curves depicting chromatic aberration of magnification, the vertical axis represents the half angle of view (ω), d represents wavelength characteristics corresponding to d-line (λ=587.56 nm), F represents wavelength characteristics corresponding to F-line (λ=486.13 nm), and C represents wavelength characteristics corresponding to C-line (λ=656.27 nm).

FIG. 3 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a second embodiment. The imaging optical system is configured to include sequentially from the object side nearest a non-depicted object, a front group G₂₁ having a positive refractive power, the diaphragm STP prescribing a given aperture, and a rear group G₂₂ having a positive refractive power. Between the rear group G₂₂ and the image plane IMG, the optical filter F and the cover glass CG are disposed sequentially from the object side. At the image plane IMG, the light receiving surface of the solid state image sensor is disposed.

The front group G₂₁ is configured to include sequentially from the object side, a negative lens L₂₁₁ and a positive lens L₂₁₂. Both surfaces of the positive lens L₂₁₂ are aspheric.

The rear group G₂₂ is configured to include sequentially from the object side, a negative lens L₂₂₁ (first lens), a positive lens L₂₂₂ (second lens), and a positive lens L₂₂₃ (third lens). The surface on the object side of the negative lens L₂₂₁ is concave. Both surfaces of the positive lens L₂₂₃ are aspheric.

Here, various types of data related to the imaging optical system according to the second embodiment are given.

f (focal length of entire imaging optical system) = 6.0 Fno. (F number) = 2.0 2ω (angle of view) = 60.0 (Lens data) r₁ = 65.8266 d₁ = 1.0116 nd₁ = 1.658 υd₁ = 50.85 r₂ = 3.8349 d₂ = 3.9183 r₃ = 5.6968 (aspheric) d₃ = 1.2437 nd₂ = 1.821 υd₂ = 42.71 r₄ = −23.2935 (aspheric) d₄ = 0.0981 r₅ = ∞ (diaphragm) d₅ = 3.1501 r₆ = −2.5463 d₆ = 0.6333 nd₃ = 2.003 υd₃ = 19.32 r₇ = −4.7523 d₇ = 0.0978 r₈ = ∞ d₈ = 2.5992 nd₄ = 1.497 υd₄ = 81.61 r₉ = −4.6138 d₉ = 0.0923 r₁₀ = 18.5992 (aspheric) d₁₀ = 2.2238 nd₅ = 1.592 υd₅ = 67.02 r₁₁ = −19.1491 (aspheric) d₁₁ = 0.1000 r₁₂ = ∞ d₁₂ = 1.0000 nd₆ = 1.516 υd₆ = 64.14 r₁₃ = ∞ d₁₃ = 5.9000 r₁₄ = ∞ d₁₄ = 0.4000 nd₇ = 1.516 υd₇ = 64.14 r₁₅ = ∞ d₁₅ = 0.1568 r₁₆ = ∞ (image plane) Constant of the cone (ε) and aspheric coefficients (A, B, C, D, E) (Third order) ε = 1, A = 9.52503 × 10⁻⁴, B = 9 .49260 × 10⁻⁵, C = −1.65311 × 10⁻⁵, D = 2.07115 × 10⁻⁶, E = −3.78499 × 10⁻⁸ (Fourth order) ε = 1, A = 3.53564 × 10⁻⁴, B = 1.16745 × 10⁻⁴, C = −3.05943 × 10⁻⁵, D = 4.71509 × 10⁻⁶, E = −2.04009 × 10⁻⁷ (Tenth order) ε = 1, A = 4.83657 × 10⁻⁴, B = −5.52837 × 10⁻⁵, C = 6.65896 × 10⁻⁶, D = −5.30466 × 10⁻⁷, E = −7.03815 × 10⁻¹⁰ (Eleventh order) ε = 1, A = 1.77671 × 10⁻³, B = −8.75517 × 10⁻⁶, C = 5.24423 × 10⁻⁶, D = −5.51386 × 10⁻⁷, E = 5.19802 × 10⁻⁹ (Values related to conditional expression (1)) θ3 (temperature-dependent variation of relative refractive index for d-line of negative lens L₂₂₁) = 6.8 θ2 (temperature-dependent variation of relative refractive index for d-line of positive lens L₂₁₂) = 6.1 |θ3/θ2| = 1.11 (Values related to conditional expression (2)) F2 (focal length of rear group G₂₂) = 12.14 f21 (focal length of negative lens L₂₂₁) = −6.39 F2/f21 = −1.90 (Values related to conditional expression (3)) F1 (focal length of front group G₂₁) = 7.77 f12 (focal length of positive lens L₂₁₂) = 5.69 F1/f12 = 1.37 (Values related to conditional expression (4)) υ22 (Abbe number for d-line of positive lens L₂₂₂ (second lens)) = 81.61 υ21 (Abbe number for d-line of negative lens L₂₂₁ (first lens)) = 19.32 υ22/υ21 = 4.22 (Values related to conditional expression (5)) f23 (focal length of positive lens L₂₂₃ (third lens)) = 16.29 F1 (focal length of front group G₂₁) = 7.77 |f23/F1| = 2.10

FIG. 4 is a diagram of various types of aberration occurring in the imaging optical system according to the second embodiment. In the diagram, for the curve depicting spherical aberration, the vertical axis represents the F number (Fno.) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the solid line depicts characteristics of the sagittal plane (S) and the broken line depicts characteristics on the meridonal plane (M). For the curve depicting distortion, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting chromatic aberration of magnification, the vertical axis represents the half angle of view (ω), d represents wavelength characteristics corresponding to d-line (λ=587.56 nm), F represents wavelength characteristics corresponding to F-line (λ=486.13 nm), and C represents wavelength characteristics corresponding to C-line (λ=656.27 nm).

FIG. 5 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a third embodiment. The imaging optical system is configured to include sequentially from the object side nearest a non-depicted object, a front group G₃₁ having a positive refractive power, the diaphragm STP prescribing a given aperture, and a rear group G₃₂ having a positive refractive power. Between the rear group G₃₂ and the image plane IMG, the optical filter F and the cover glass CG are disposed sequentially from the object side. At the image plane IMG, the light receiving surface of the solid state image sensor is disposed.

The front group G₃₁ is configured to include sequentially from the object side, a negative lens L₃₁₁ and a positive lens L₃₁₂. Both surfaces of the positive lens L₃₁₂ are aspheric.

The rear group G₃₂ is configured to include sequentially from the object side, a negative lens L₃₂₁ (first lens), a positive lens L₃₂₂ (second lens), and a positive lens L₃₂₃ (third lens). The surface on the object side of the negative lens L₃₂₁ is concave. Both surfaces of the positive lens L₃₂₃ are aspheric.

Here, various types of data related to the imaging optical system according to the third embodiment are given.

f (focal length of entire imaging optical system) = 6.0 Fno. (F number) = 2.0 2ω (angle of view) = 60.0 (Lens data) r₁ = −27.1372 d₁ = 1.5009 nd₁ = 1.658 υd₁ = 50.85 r₂ = 5.7712 d₂ = 5.7163 r₃ = 5.5663 (aspheric) d₃ = 1.5947 nd₂ = 1.773 υd₂ = 49.50 r₄ = −30.5705 (aspheric) d₄ = 0.3760 r₅ = ∞ (diaphragm) d₅ = 3.3979 r₆ = −2.6664 d₆ = 0.5925 nd₃ = 2.104 υd₃ = 17.02 r₇ = −4.2419 d₇ = 0.0990 r₈ = ∞ d₈ = 4.8050 nd₄ = 1.437 υd₄ = 95.01 r₉ = −4.9880 d₉ = 0.0762 r₁₀ = −40.4531 (aspheric) d₁₀ = 1.4934 nd₅ = 1.592 υd₅ = 67.02 r₁₁ = −8.9340 (aspheric) d₁₁ = 0.1000 r₁₂ = ∞ d₁₂ = 1.0000 nd₆ = 1.516 υd₆ = 64.14 r₁₃ = ∞ d₁₃ = 5.5000 r₁₄ = ∞ d₁₄ = 0.4000 nd₇ = 1.516 υd₇ = 64.14 r₁₅ = ∞ d₁₅ = 0.1524 r₁₆ = ∞ (image plane) Constant of the cone (ε) and aspheric coefficients (A, B, C, D, E) (Third order) ε = 1, A = 7.45640 × 10⁻⁴, B = 2.52063 × 10⁻⁵, C = −4.16410 × 10⁻⁷, D = 3.01641 × 10⁻⁸, E = 2.15863 × 10⁻⁸ (Fourth order) ε = 1, A = 9.38975 × 10⁻⁴, B = 3.52889 × 10⁻⁵, C = −7.37831 × 10⁻⁶, D = 1.15453 × 10⁻⁶, E = −3.94475 × 10⁻⁸ (Tenth order) ε = 1, A = 1.70005 × 10⁻⁴, B = −4.72281 × 10⁻⁵, C = 9.51867 × 10⁻⁶, D = −4.72118 × 10⁻⁷, E = −3.49177 × 10⁻⁹ (Eleventh order) ε = 1, A = 1.39589 × 10⁻³, B = −3.85409 × 10⁻⁵, C = 9.95823 × 10⁻⁶, D = −4.53243 × 10⁻⁷, E = −1.54170 × 10⁻⁹ (Values related to conditional expression (1)) θ3 (temperature-dependent variation of relative refractive index for d-line of negative lens L₃₂₁) = 9.6 θ2 (temperature-dependent variation of relative refractive index for d-line of positive lens L₃₁₂) = 5.5 |θ3/θ2| = 1.75 (Values related to conditional expression (2)) F2 (focal length of rear group G₃₂) = 10.89 f21 (focal length of negative lens L₃₂₁) = −8.10 F2/f21 = −1.34 (Values related to conditional expression (3)) F1 (focal length of front group G₃₁) = 6.40 f12 (focal length of positive lens L₃₁₂) = 6.22 F1/f12 = 1.03 (Values related to conditional expression (4)) υ22 (Abbe number for d-line of positive lens L₃₂₂ (second lens)) = 95.01 υ21 (Abbe number for d-line of negative lens L₃₂₁ (first lens)) = 17.02 υ22/υ21 = 5.59 (Values related to conditional expression (5)) f23 (focal length of positive lens L₃₂₃ (third lens)) = 19.03 F1 (focal length of front group G₃₁) = 6.40 |f23/F1| = 2.98

FIG. 6 is a diagram of various types of aberration occurring in the imaging optical system according to the third embodiment. In the diagram, for the curve depicting spherical aberration, the vertical axis represents the F number (Fno.) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the solid line depicts characteristics of the sagittal plane (S) and the broken line depicts characteristics on the meridonal plane (M). For the curve depicting distortion, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting chromatic aberration of magnification, the vertical axis represents the half angle of view (ω), d represents wavelength characteristics corresponding to d-line (λ=587.56 nm), F represents wavelength characteristics corresponding to F-line (λ=486.13 nm), and C represents wavelength characteristics corresponding to C-line (λ=656.27 nm).

FIG. 7 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a fourth embodiment. The imaging optical system is configured to include sequentially from the object side nearest a non-depicted object, a front group G₄₁ having a positive refractive power, the diaphragm STP prescribing a given aperture, and a rear group G₄₂ having a positive refractive power. Between the rear group G₄₂ and the image plane IMG, the optical filter F and the cover glass CG are disposed sequentially from the object side. At the image plane IMG, the light receiving surface of the solid state image sensor is disposed.

The front group G₄₁ is configured to include sequentially from the object side, a negative lens L₄₁₁ and a positive lens L₄₁₂. Both surfaces of the positive lens L₄₁₂ are aspheric.

The rear group G₄₂ is configured to include sequentially from the object side, a negative lens L₄₂₁ (first lens), a positive lens L₄₂₂ (second lens), and a positive lens L₄₂₃ (third lens). The surface on the object side of the negative lens L₄₂₁ is concave. Both surfaces of the positive lens L₄₂₃ are aspheric.

Here, various types of data related to the imaging optical system according to the fourth embodiment are given.

f (focal length of entire imaging optical system) = 6.0 Fno. (F number) = 2.0 2ω (angle of view) = 60.0 (Lens data) r₁ = 41.1116 d₁ = 0.8519 nd₁ = 1.658 υd₁ = 50.85 r₂ = 4.2164 d₂ = 4.8778 r₃ = 6.1042 (aspheric) d₃ = 1.1727 nd₂ = 1.851 υd₂ = 40.10 r₄ = −35.0171 (aspheric) d₄ = 0.0983 r₅ = ∞ (diaphragm) d₅ = 3.0647 r₆ = −2.6567 d₆ = 0.5972 nd₃ = 2.003 υd₃ = 19.32 r₇ = −5.3099 d₇ = 0.2841 r₈ = −24.7903 d₈ = 1.6508 nd₄ = 1.593 υd₄ = 68.62 r₉ = −5.0088 d₉ = 0.3589 r₁₀ = 128.1394 (aspheric) d₁₀ = 2.3479 nd₅ = 1.553 υd₅ = 71.68 r₁₁ = −7.2545 (aspheric) d₁₁ = 0.1000 r₁₂ = ∞ d₁₂ = 1.0000 nd₆ = 1.516 υd₆ = 64.14 r₁₃ = ∞ d₁₃ = 6.5000 r₁₄ = ∞ d₁₄ = 0.4000 nd₇ = 1.516 υd₇ = 64.14 r₁₅ = ∞ d₁₅ = 0.1918 r₁₆ = ∞ (image plane) Constant of the cone (ε) and aspheric coefficients (A, B, C, D, E) (Third order) ε = 1, A = 1.20488 × 10⁻³, B = 7.47898 × 10⁻⁵, C = −8.79387 × 10⁻⁶, D = 5.62957 × 10⁻⁷, E = −1.28602 × 10⁻⁹ (Fourth order) ε = 1, A = 6.92038 × 10⁻⁴, B = 5.12202 × 10⁻⁵, C = −1.19177 × 10⁻⁵, D = 8.34322 × 10⁻⁷, E = −1.81539 × 10⁻⁸ (Tenth order) ε = 1, A = 1.67025 × 10⁻⁴, B = −5.83690 × 10⁻⁷, C = 1.23322 × 10⁻⁵, D = −9.90837 × 10⁻⁷, E = 2.66409 × 10⁻⁸ (Eleventh order) ε = 1, A = 1.33035 × 10⁻³, B = 5.03731 × 10⁻⁵, C = 1.45378 × 10⁻⁶, D = 1.30131 × 10⁻⁷, E = −6.79719 × 10⁻⁹ (Values related to conditional expression (1)) θ3 (temperature-dependent variation of relative refractive index for d-line of negative lens L₄₂₁) = 6.8 θ2 (temperature-dependent variation of relative refractive index for d-line of positive lens L₄₁₂) = 7.5 |θ3/θ2| = 0.91 (Values related to conditional expression (2)) F2 (focal length of rear group G₄₂) = 12.48 f21 (focal length of negative lens L₄₂₁) = −5.98 F2/f21 = −2.09 (Values related to conditional expression (3)) F1 (focal length of front group G₄₁) = 7.52 f12 (focal length of positive lens L₄₁₂) = 6.19 F1/f12 = 1.21 (Values related to conditional expression (4)) υ22 (Abbe number for d-line of positive lens L₄₂₂ (second lens)) = 68.62 υ21 (Abbe number for d-line of negative lens L₄₂₁ (first lens)) = 19.32 υ22/υ21 = 3.71 (Values related to conditional expression (5)) f23 (focal length of positive lens L₄₂₃ (third lens)) = 12.49 F1 (focal length of front group G₄₁) = 7.52 |f23/F1| = 1.66

FIG. 8 is a diagram of various types of aberration occurring in the imaging optical system according to the fourth embodiment. In the diagram, for the curve depicting spherical aberration, the vertical axis represents the F number (Fno.) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the vertical axis represents the half angle of view (m) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the solid line depicts characteristics of the sagittal plane (S) and the broken line depicts characteristics on the meridonal plane (M). For the curve depicting distortion, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting chromatic aberration of magnification, the vertical axis represents the half angle of view (ω), d represents wavelength characteristics corresponding to d-line (λ=587.56 nm), F represents wavelength characteristics corresponding to F-line (λ=486.13 nm), and C represents wavelength characteristics corresponding to C-line (λ=656.27 nm).

FIG. 9 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a fifth embodiment. The imaging optical system is configured to include sequentially from the object side nearest a non-depicted object, a front group G₅₁ having a positive refractive power, the diaphragm STP prescribing a given aperture, and a rear group G₅₂ having a positive refractive power. Between the rear group G₅₂ and the image plane IMG, the optical filter F and the cover glass CG are disposed sequentially from the object side. At the image plane IMG, the light receiving surface of the solid state image sensor is disposed.

The front group G₅₁ is configured to include sequentially from the object side, a negative lens L₅₁₁ and a positive lens L₅₁₂. Both surfaces of the positive lens L₅₁₂ are aspheric.

The rear group G₅₂ is configured to include sequentially from the object side, a negative lens L₅₂₁ (first lens), a positive lens L₅₂₂ (second lens), and a positive lens L₅₂₃ (third lens). The surface on the object side of the negative lens L₅₂₁ is concave. Both surfaces of the positive lens L₅₂₃ are aspheric.

Here, various types of data related to the imaging optical system according to the fifth embodiment are given.

f (focal length of entire imaging optical system) = 6.0 Fno. (F number) = 2.0 2ω (angle of view) = 60.0 (Lens data) r₁ = −32.0676 d₁ = 0.5995 nd₁ = 1.658 υd₁ = 50.85 r₂ = 3.7450 d₂ = 3.0398 r₃ = 5.3867 (aspheric) d₃ = 1.8428 nd₂ = 1.755 υd₂ = 51.16 r₄ = −10.9413 (aspheric) d₄ = 0.2050 r₅ = ∞ (diaphragm) d₅ = 3.308 r₆ = −2.4348 d₆ = 0.5732 nd₃ = 2.003 υd₃ = 19.32 r₇ = −4.0218 d₇ = 0.1002 r₈ = ∞ d₈ = 2.6507 nd₄ = 1.437 υd₄ = 95.10 r₉ = −4.2484 d₉ = 0.0874 r₁₀ = −149.7661 (aspheric) d₁₀ = 1.4626 nd₅ = 1.592 υd₅ = 67.02 r₁₁ = −10.2978 (aspheric) d₁₁ = 0.1000 r₁₂ = ∞ d₁₂ = 1.0000 nd₆ = 1.516 υd₆ = 64.14 r₁₃ = ∞ d₁₃ = 5.6000 r₁₄ = ∞ d₁₄ = 0.4000 nd₇ = 1.516 υd₇ = 64.14 r₁₅ = ∞ d₁₅ = 0.1179 r₁₆ = ∞ (image plane) Constant of the cone (ε) and aspheric coefficients (A, B, C, D, E) (Third order) ε = 1, A = 4.08337 × 10⁻⁴, B = 1.71269 × 10⁻⁵, C = −3.98433 × 10⁻⁶, D = 1.90505 × 10⁻⁷, E = 7.03809 × 10⁻⁸ (Fourth order) ε = 1, A = 3.66795 × 10⁻⁴, B = 8.40142 × 10⁻⁵, C = −1.67953 × 10⁻⁵, D = 2.42398 × 10⁻⁶, E = −2.47079 × 10⁻⁸ (Tenth order) ε = 1, A = 1.38636 × 10⁻³, B = −1.97184 × 10⁻⁴, C = 4.68211 × 10⁻⁵, D = −4.17903 × 10⁻⁶, E = 1.04142 × 10⁻⁷ (Eleventh order) ε = 1, A = 2.79141 × 10⁻³, B = −2.00511 × 10⁻⁴, C = 5.41425 × 10⁻⁵, D = −4.57761 × 10⁻⁶, E = 1.15891 × 10⁻⁷ (Values related to conditional expression (1)) θ3 (temperature-dependent variation of relative refractive index for d-line of negative lens L₅₂₁) = 6.8 θ2 (temperature-dependent variation of relative refractive index for d-line of positive lens L₅₁₂) = 4.7 |θ3/θ2| = 1.45 (Values related to conditional expression (2)) F2 (focal length of rear group G₅₂) = 12.19 f21 (focal length of negative lens L₅₂₁) = −7.51 F2/f21 = −1.62 (Values related to conditional expression (3)) F1 (focal length of front group G₅₁) = 7.31 f12 (focal length of positive lens L₅₁₂) = 5.02 F1/f12 = 1.46 (Values related to conditional expression (4)) υ22 (Abbe number for d-line of positive lens L₅₂₂ (second lens)) = 95.10 υ21 (Abbe number for d-line of negative lens L₅₂₁ (first lens)) = 19.32 υ22/υ21 = 4.92 (Values related to conditional expression (5)) f23 (focal length of positive lens L₅₂₃ (third lens)) = 18.61 F1 (focal length of front group G₅₁) = 7.31 |f23/F1| = 2.54

FIG. 10 is a diagram of various types of aberration occurring in the imaging optical system according to the fifth embodiment. In the diagram, for the curve depicting spherical aberration, the vertical axis represents the F number (Fno.) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the solid line depicts characteristics of the sagittal plane (S) and the broken line depicts characteristics on the meridonal plane (M). For the curve depicting distortion, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting chromatic aberration of magnification, the vertical axis represents the half angle of view (ω), d represents wavelength characteristics corresponding to d-line (λ=587.56 nm), F represents wavelength characteristics corresponding to F-line (λ=486.13 nm), and C represents wavelength characteristics corresponding to C-line (λ=656.27 nm).

FIG. 11 is a diagram depicting, along the optical axis, a configuration of the imaging optical system according to a sixth embodiment. The imaging optical system is configured to include sequentially from the object side nearest a non-depicted object, a front group G₆₁ having a positive refractive power, the diaphragm STP prescribing a given aperture, and a rear group G₆₂ having a positive refractive power. Between the rear group G₆₂ and the image plane IMG, the optical filter F and the cover glass CG are disposed sequentially from the object side. At the image plane IMG, the light receiving surface of the solid state image sensor is disposed.

The front group G₆₁ is configured to include sequentially from the object side, a negative lens L₆₁₁ and a positive lens L₆₁₂. Both surfaces of the positive lens L₆₁₂ are aspheric.

The rear group G₆₂ is configured to include sequentially from the object side, a negative lens L₆₂₁ (first lens), a positive lens L₆₂₂ (second lens), and a positive lens L₆₂₃ (third lens). The surface on the object side of the negative lens L₆₂₁ is concave. Both surfaces of the positive lens L₅₂₃ are aspheric.

Here, various types of data related to the imaging optical system according to the sixth embodiment are given.

f (focal length of entire imaging optical system) = 6.0 Fno. (F number) = 2.0 2ω (angle of view) = 60.0 (Lens data) r₁ = 14.8694 d₁ = 1.5052 nd₁ = 1.658 υd₁ = 50.85 r₂ = 2.7207 d₂ = 2.4484 r₃ = 5.0757 (aspheric) d₃ = 1.8837 nd₂ = 1.851 υd₂ = 40.10 r₄ = −14.0158 (aspheric) d₄ = 0.0959 r₅ = ∞ (diaphragm) d₅ = 2.0386 r₆ = −2.4234 d₆ = 0.5440 nd₃ = 1.923 υd₃ = 18.90 r₇ = −5.7476 d₇ = 0.3976 r₈ = −5253710.9 d₈ = 1.6284 nd₄ = 1.593 υd₄ = 68.62 r₉ = −6.7356 d₉ = 0.0893 r₁₀ = 11.7208 (aspheric) d₁₀ = 1.6544 nd₅ = 1.592 υd₅ = 67.02 r₁₁ = −8.8918 (aspheric) d₁₁ = 0.1000 r₁₂ = ∞ d₁₂ = 1.0000 nd₆ = 1.516 υd₆ = 64.14 r₁₃ = ∞ d₁₃ = 6.0500 r₁₄ = ∞ d₁₄ = 0.4000 nd₇ = 1.516 υd₇ = 64.14 r₁₅ = ∞ d₁₅ = 0.1471 r₁₆ = ∞ (image plane) Constant of the cone (ε) and aspheric coefficients (A, B, C, D, E) (Third order) ε = 1, A = 8.25004 × 10⁻⁴, B = 2.62772 × 10⁻⁵, C = −1.35690 × 10⁻⁵, D = −2.97073 × 10⁻⁷, E = −1.68159 × 10⁻⁸ (Fourth order) ε = 1, A = −1.94258 × 10⁻³, B = 1.38913 × 10⁻⁵, C = −9.56376 × 10⁻⁶, D = 5.67059 × 10⁻⁷, E = −2.73708 × 10⁻⁹ (Tenth order) ε = 1, A = 2.32775 × 10⁻⁴, B = 6.07502 × 10⁻⁵, C = 4.07910 × 10⁻⁶, D = −7.29772 × 10⁻⁷, E = 1.14943 × 10⁻⁸ (Eleventh order) ε = 1, A = 2.42461 × 10⁻³, B = 7.21766 × 10⁻⁵, C = 5.52775 × 10⁻⁶, D = −6.96671 × 10⁻⁷, E = 4.62476 × 10⁻⁹ (Values related to conditional expression (1)) θ3 (temperature-dependent variation of relative refractive index for d-line of negative lens L₆₂₁) = 2.1 θ2 (temperature-dependent variation of relative refractive index for d-line of positive lens L₆₁₂) = 7.5 |θ3/θ2| = 0.28 (Values related to conditional expression (2)) F2 (focal length of rear group G₆₂) = 11.79 f21 (focal length of negative lens L₆₂₁) = −4.93 F2/f21 = −2.39 (Values related to conditional expression (3)) F1 (focal length of front group G₆₁) = 7.50 f12 (focal length of positive lens L₆₁₂) = 4.58 F1/f12 = 1.64 (Values related to conditional expression (4)) υ22 (Abbe number for d-line of positive lens L₆₂₂ (second lens)) = 68.62 υ21 (Abbe number for d-line of negative lens L₆₂₁ (first lens)) = 18.90 υ22/υ21 = 3.62 (Values related to conditional expression (5)) f23 (focal length of positive lens L₆₂₃ (third lens)) = 8.80 F1 (focal length of front group G₆₁) = 7.50 |f23/F1| = 1.17

FIG. 12 is a diagram of various types of aberration occurring in the imaging optical system according to the sixth embodiment. In the diagram, for the curve depicting spherical aberration, the vertical axis represents the F number (Fno.) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting astigmatism, the solid line depicts characteristics of the sagittal plane (S) and the broken line depicts characteristics on the meridonal plane (M). For the curve depicting distortion, the vertical axis represents the half angle of view (ω) and wavelength characteristics corresponding to d-line (λ=587.56 nm) are depicted. For curves depicting chromatic aberration of magnification, the vertical axis represents the half angle of view (ω), d represents wavelength characteristics corresponding to d-line (λ=587.56 nm), F represents wavelength characteristics corresponding to F-line (λ=486.13 nm), and C represents wavelength characteristics corresponding to C-line (λ=656.27 nm).

Among the values for each of the embodiments, r₁, r₂, . . . indicate the radius of curvature of lens surfaces, aperture surfaces, etc.; d₁, d₂, . . . indicate the thickness of the lenses, the aperture, etc. or the interval between the surfaces thereof; nd₁, nd₂, . . . indicate the refraction index of the lenses with respect to the d-line (λ=546.074 nm); and υ₁, υ₂, . . . indicate the Abbe number for the d-line (λ=587.56 nm) of the lenses. Lengths are indicated in units of “mm”; and angles are indicated in “degrees”.

In the embodiments, X, which represents the amount of aspheric sag from the surface apex on the optical axis (the direction of the image plane being positive), is expressed by the equation below; where, H represents a distance from the optical axis toward the outer diameter of the lens, R represents paraxial radius of curvature, s represents the constant of the cone, and A, B, C, D, and E respectively represent fourth, sixth, eighth, tenth, and twelfth order aspheric coefficients.

$\begin{matrix} {X = {\frac{H^{2}/R}{1 + \sqrt{1 - \left( {{zH}^{2}/R^{2}} \right)}} + {AH}^{4} + {BH}^{6} + {CH}^{8} + {DH}^{10} + {EH}^{12}}} & \lbrack 1\rbrack \end{matrix}$

As described, the imaging optical system of the embodiments satisfies conditional expression (1), whereby the temperature-dependent variation of the relative refractive index of the lens disposed at a position that most easily influences drops in resolution consequent to temperature variation is properly set, enabling high resolution to be maintained over a wide temperature range, from low temperatures to high temperatures. In particular, satisfying conditional expression (1) enables focus error of the optical system to be suppressed during high temperatures (up to 105 degrees C.) and low temperatures (down to −40 degrees C.). Therefore, without problems, the imaging optical system of the embodiments can be used on vehicle cameras provided in vehicles, which are subject to high summer temperatures, surveillance cameras installed outdoors where changes in environmental temperature is significant, etc.

Further satisfying conditional expressions (2) and (3) enables an imaging optical system having a deep depth of focus to be realized. If the depth of focus is deep, the occurrence of focus error is suppressed during extreme temperatures and high resolution can be maintained over a wide temperature range, from low temperatures to high temperatures. Further, if the depth of focus is deep, image quality is not easily affected by shifts in lens centers occurring at the time of product manufacturing, enabling favorable image quality to be maintained.

Sequentially satisfying conditional expression (4) and conditional expression (5) is effective in preventing drops in peripheral illumination and in correcting chromatic aberration of magnification, astigmatism, and distortion, essential for maintaining high resolution. In the imaging optical system of the embodiments, lenses having properly shaped aspheric surfaces are disposed, enabling greater aberration correction performance with fewer lenses.

As described, the imaging optical system according to the present invention is useful for imaging apparatuses from which high resolution is demanded over a wide temperature range, from low temperatures to high temperatures, and is particularly suitable for vehicle cameras and surveillance camera used under conditions of extreme environmental temperature changes.

Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-250831, filed on Dec. 11, 2014, the entire contents of which are incorporated herein by reference. 

What is claimed is:
 1. An imaging optical system comprising sequentially from an object side: a front group configured to include a positive lens disposed at a position nearest a diaphragm; the diaphragm; and a rear group configured to include a negative lens disposed at a position nearest the diaphragm, wherein the imaging optical system satisfies a conditional expression (1) 0.27≦|θ3/θ2|≦1.80, where θ3 represents temperature-dependent variation of relative refractive index for d-line of the negative lens and θ2 represents temperature-dependent variation of relative refractive index for d-line of the positive lens.
 2. The imaging optical system according to claim 1, wherein the imaging optical system satisfies a conditional expression (2) −2.4≦F2/f21≦−1.3 and a conditional expression (3) 1.00≦F1/f12≦1.65, where F2 represents a focal length of the rear group, f21 represents a focal length of the negative lens, F1 represents a focal length of the front group, and f12 represents a focal length of the positive lens.
 3. The imaging optical system according to claim 1, wherein the rear group is configured to include sequentially from the object side, a first lens, a second lens, and a third lens, and the imaging optical system satisfies a conditional expression (4) 3.6≦υ22/υ21≦5.6, where υ22 represents an Abbe number for d-line of the second lens and υ21 represents an Abbe number for d-line of the first lens.
 4. The imaging optical system according to claim 1, wherein the rear group is configured to include sequentially from the object side, a first lens, a second lens, and a third lens, and the imaging optical system satisfies a conditional expression (5) 1.15≦|f23/F1|≦3.0, where f23 represents a focal length of the third lens and F1 represents a focal length of the front group. 